The invention relates to a method of monitoring a web in a machine processing such web and more particularly a printing press in which a monitoring beam is directed onto the web, and part of the beam returned from the web is received and in which the interval between the transmission of the monitoring beam and receiving the returned part thereof is measured and in the case of such interval exceeding a predetermined tolerated range a control pulse is produced. In accordance with a further aspect the invention relates to an apparatus for performing such method comprising at least one sensor arranged on the path of the web and having on the one hand a transmitter for a monitoring beam directed onto the web and on the other hand a receiver for a reflected part of the beam, which receiver is connected with a signal processing means by means of which the interval between the transmission of the monitoring beam and the reception thereof may be measured and in the case of such interval exceeding a predetermined tolerated range a control pulse is produced.
Up till now operation has been performed with a permanently set tolerated range, which is accordingly effective for all speeds of the web. As long as the web is present within this tolerated range no action is taken. As soon as there ceases to be any web present within the predetermined tolerated range a control signal is produced, which generally is responsible for turning off the machine and/or catching the web and/or severing the web. A disadvantage in this respect is that the web does not run equally smoothly at every speed and often tends to flutter. In the case of a paper web to be printed in a printing press this will generally be the case when the web is being drawn in and while running the printing press up to operating speed. In this respect even owing to its flutter which in order to avoid slow response may not be excessive, there is a danger of the web getting out of the permissible tolerated range, and the consequence of this is that a spurious pulse indicating a break in the web will be produced without such break in fact having occurred.
Although a greater tolerated range would prevent spurious signals from being produced during drawing in the web and running up to speed, during normal operation at full speed it would be responsible for a comparatively slow response, this being undesired. Therefore so far compromises nave been necessary which impair reliability during drawing in the web and running up to operational speed and during normal operation do not provide sufficient safety.